Cell cover with weakened areas to relieve explosions

ABSTRACT

DESCRIBES A CELL COVER FOR ALKALI CHLORINE CELLS, PRIMARILY FOR USE WITH DIMENSIONALLY STABLE ANODES, BUT ALSO USEFUL WITH GRAPHITE ANODES, WHCIH CELL COVER IS PROVIDED WITH WEAKENED AREAS, IN ACCESSIBLE PLACE IN THE CELL COVER, THROUGH WHICH THE FORCE OF EXPLOSIONS CAN BE VENTED AND WHICH WEAKENED AREAS WHEN RUPTURED BY AN EXPLOSION CAN BE EASILY REPAIRED.

June 27, 1972 v. DE NORA EI'AL CELL COVER WITH WEAKENED AREAS TO RELIEVE EXPLOSIONS Filed NOV. 28, 1969 2 Sheets-Sheet 1 INVENT June 27, 1972 V. DE NORA EI'AL CELL COVER WITH WEAKENED AREAS TO RELIEVE EXPLOSIONS Filed Nov. 28, 1969 2 Sheets-Sheet 2 l A {55 Won 5 V L- L 7 r 5 INVENTORS RICHARD E. LOFTFIELD Y VITTORIO DINORA United States Patent 015cc US. Cl. 204-99 8 Claims ABSTRACT OF THE DISCLOSURE Describes a cell cover for alkali chlorine cells, primarily for use with dimensionally stable anodes, but also useful with graphite anodes, which cell cover is provided with weakened areas, in accessible place in the cell cover, through which the force of explosions can be vented and which weakened areas when ruptured by an explosion can be easily repaired.

This invention relates to cell covers for electrolytic mercury cell for the electrolysis of salt solutions, molten salts, etc., in which the cathode is a flowing mercury layer. When dimensionally stable anodes such as oxide coated or platinum plated titanium anodes are used in such cells, the gap between the anodes and the flowing mercury cathode is maintained as small as possible, and

accidental contacts between the flowing mercury cathode and the anodes is more frequent than in cells equipped with graphite anodes in which a wider anode gap is used. However, these contacts are generally quickly broken and short circuits of long duration are not formed. On the contrary, small explosions or poppings occur in such cells equipped with dimensionally stable anodes and these small explosions are suflicient in many instances to tear a hole in flexible cell covers of the type described in Us. "Pat. No. 2,958,635. These holes are often in inaccessible places, such as beneath other parts of the cell and are often difficult to reach for repair and/or replacement of the cell cover.

The term dimensionally stable anodes as used herein is intended to describe metal anodes whose dimensions do not change in use. They are usually formed of a core 'of a valve metal, such as titanium or tantalum, having a conductive, electrocatalytic coating on said core which conducts current from the core to the electrolyte and catalyzes chlorine discharge from the anode. The coatings may be of a platinum group metal, mixed oxides of valve metals and platinum group metals spinel coatings with electrocatalytic ingredients, or any other coating which will conduct current from the valve metal base and catalyze discharge of chlorine or any other gaseous electrolysis product from the anode.

The holes resulting from such explosions are often in sections of the cell cover which are under the busbars or under the anode supports or in otherwise inaccessible portions of the cell cover and often require dismantling the cell to repair or replace the cell cover with consequent labor cost and loss of production. As cells equipped with dimensionally stable anodes otherwise require less frequent dismantling for maintenance purposes than cells equipped with graphite anodes, it is desirable to provide for quick repair of torn cell covers without dismantling the cell to replace a torn'cover. When rigid or part-rigid cell covers, such as steel or hard rubber covers, are used with dimensionally stable anodes, these small explosions often damage the coves beyond repair, so that complete replacement is necessary.

Patented June 27, 1972 The exact cause of these small explosions or poppings 1s not known, but we believe that they are caused by short circuits between the dimensionally stable anodes and the mercury cathode which trigger an explosion of the hydrogen-chlorine mixture adjacent the anode at which the short circuit occurs. Unlike explosions in chlorine cells with graphite anodes, these explosions are much more frequent (because of the reduced anode gap) and smaller.

One of the objects of this invention is to provide covers for cells equipped with dimensionally stable anodes which will relieve or vent the force of an explosion with a minimum of damage to the cell or cell cover and which damage canbe quickly repaired so that the cell is out of operation a mlnlmum amount of time.

Another object of this invention is to provide weakened portions in the cell covers through which the force of the explosions can be vented, and which weakened portions are readily accessible and can be easily repaired after an explosion.

Other objects and advantages will become apparent as this description proceeds.

While the invention is applicable to cells equipped with rigid cell covers, or with part-rigid and part-flexible cell covers, it will be described primarily with reference to electrolytic cells equipped with flexible cell covers as described in said US. Pat. No. 2,958,635.

Referring now to the drawings which show preferred embodiments of the invention:

FIG. 1 is a representative perspective elevation, with parts broken away, of an electrolytic cell embodying this lnvention, with parts not essential to the understanding of this invention omitted;

FIG. 2 is a cross sectional view approximately along thetline 22 of FIG. 1, showing one form of explosion ven 1 FIG. 3 is an enlarged detail perspective of one end of two anode connections, showing weakened areas in the cell cover;

FIG. 4 is a cross sectional view of a cell, taken substantially'along the line 44 of FIG. 1;

' FIG. 5 is a plan view along the line 55 of FIG. 4;

FIG. 6 is a sectional view of a portion of a cell cover showing weakened areas to readily vent the 'force of explosions;

FIG. 7 is a part sectional view of another embodiment of the invention;

FIG. 8 is a cross sectional view of a safety vent for a rigid cell cover;

FIG. 9 is a view similar to FIG. 8, showing a vent construction applicable to either a rigid or flexible cell cover;

FIGS. 10 and 11 are views of a balloon-like plug which may be used to plug openings in a flexible or rigid cell cover;

FIG. 12 shows the normal side connection between a flexible cell cover and a side or end wall of the cell; and

FIG. 13 shows a loose connection between a portion of the cover and the side walls through which the force of an explosion may be vented.

Referring to the drawings, the metallic base 1 of a horizontal mercury cell is usually formed from steel I-beam sides 1A and a flat steel bottom 1B. This body is lined with concrete 6 and the sides and bottom of the cell are lined with an insulating layer of a stone material 5 or other insulating material, such as a coating of resin. At suitable intervals, the bottom of the cell is crossed by steel I-beam sections 9 which are connected to the bottom 1B and which act as electrical connectors between the mercury flowing along the cell floor and the negative bus bar 12. Alternatively, the bottom of the cell may be a steel plate, connected to the bottom 1B and to the negative bus bar 12. The anodes 8 comprise flat horizontal titanium or other valve metal plates or bases 8a coated with a conductive electrocatalytic coating (not specifically shown), such as a platinum group metal or a semi-conducting oxide coating of titanium and ruthenium oxide or other oxides which are conducting and have electrocatalytic properties. The anode plates 8a ar attached to vertical cylindrical copper lead-in rods 27 which are surrounded with titanium or other protective sleeves 270 (FIG. 2) inside the cell and are connected by nuts 30 for attachment to the anode lifting frame structures 15-20 and to the positive bus bars 11.

The dimensionally stable anodes 8 are formed of solid or foraminous plates 8a of a valve metal core, such as titanium or tantalum, covered with a conductive electrocatalytic coating such as a platinum group metal or a mixed oxide coating of titanium and ruthenium oxides or other conductive electrocatalytic coatings. The plates 8a are connected, preferably by welding, to secondary titanium conductor bars 8b which, in turn, are connected to primary titanium conductor bars 8c, electrically connected by collars 8d to the copper lead-in connectors 27. The lead-in connectors 27 are surrounded inside the cell with titanium sleeves 270 (FIG. 2) which are sealed at each end to prevent the cell gases and liquors from contacting the copper lead-ins. The plates 8a are spaced a short distanoe, 1.5 to 3.5 mm., from the flowing mercury cathode 9a (FIG. 2), flowing along the top of the cell base 1. The short circuits which cause the small explosions or poppings" form between the mercury surface and the dimensionally stable anode plates 8a when ripples or other unevenness in the mercury causes a temporary contact between the mercury and the anodes. The smaller the anode gap, the more frequent these contacts occur.

The anodes 8 are suspended and supported on the lead-ins 27 from a metal frame spider 15 which consists of transverse bars 20 secured to longitudinal I-beams 21. The transverse bars 20 are adjustably supported on posts 19 which rest upon or are secured to the sides 1A of the cell trough or rest upon rigid supports outside the cell trough. Adjusting means such as screw threaded nuts 22 are provided on the posts or adjusting jacks 19, so that the height of the spider 15 and of the anodes supported therefrom can be adjusted relative to the cell trough. Suitable means, such as eye hooks 32,.are provided on the frame 15 for lifting the whole frame off the cell, with the anodes suspended therefrom, when desired.

Anode support bars 40 are attached to the I-beams 21 at spaced intervals as indicated in FIG. 1 and the anodes 8 are adjustably suspended by the lead-ins 27 from the bars 40 by adjustable nuts 30 and 31. Bars 40 are connected to the I-beams 21 of the frame 15 by welding or the like. It is mainly necessary that frame 15 from which the anodes 8 are suspended be supported in a fixed position with respect to the cell trough, and be adjustable relative thereto.

FIG. 1 illustrates a flexible cell cover which is divided into three sections A, B and C by transverse dividing bars A and B extending across the top of the cell trough (but not to the bottom of the trough) and the anode supporting frames or spiders 15 are likewise divided into three separate sections, so that each section may be lifted slightly or adjusted separate from the other section.

A flexible cover 17 made of sheet rubber or other sheet plastic material, such as described, for example, in US. Pats. Nos. 2,998,374 and 3,450,621, through which the copper lead-ins 27 extends, closes the top of the cell and is fastened to the sides of the cell and the transverse dividing bars A and B' by means of bolted strips 24 which may have loose sections 24a as hereafter described. Other suitable clamping means can be provided. Suitable gaskets 27b are provided between lead-ins 27 and the flexible cover 17 to prevent escape of gases, and chlorine outlets 44 are provided through the flexible cover 17 of the cell above the brine level.

Copper bus bars 12 provide electric connections for the cathode through the bottom of the plate 1B and through metallic I-beams 9 which are connected to the base plate 1B and which are embedded in the lining of the cell and come into contact with the mercury flowing along the bottom of the cell (see FIG. 4) or through a steel plate forming the bottom of the cell trough. Electric connection is provided to the anodes 8 through the bus bars 11 which are further attached to the electric cross connections 16 which are attached to the lead-ins 27 of each anode by nuts 30a.

With this arrangement, all of the anodes attached to the frame 15 may be simultaneously adjusted through the adjusting jacks 19 and transverse bars 20 without disturbance of the flexible cover 17. The anodes may also be individually adjusted upwardly or downwardly by adjusting the nuts 30 and 30a on the lead-in 27.It is thus possible to maintain the proper distance for the most efficient operation between the lower face of the anodes 8 and the mercury cathode flowing along the bottom of the cell.

FIG. 2 shows in greater detail the anode electrical leadins 27 and bus bar connections 16 and how the flexible cover 17 is clamped between gaskets 27b and the collars around the titanium sleeves 27c which surround the leadins 27 inside the cell.

In the operation of a cell of the type shown in FIG. 1, mercury is introduced through conduit 10 into compartment 43 and passes under weir 43A into the cell compartment. The mercury flow is from left to right in FIG. 1. A sodium chloride electrolyte or brine is introduced into the cell through inlet 41 at the end of the cell adjacent decomposer 2. The mercury and brine flow toward the opposite end of the cell during which the brine is electrolyzed. The approximate brine or electrolyte level is indicated by the line E in FIG. 4. Chlorine is withdrawn through the chlorine outlets 44 and depleted brine is withdrawn at the lower end of the cell at outlet 42. Amalgam passes beneath weir 45 into open compartment 46 and is withdrawn by conduit 33, passing through trap 34 to decomposer 2. In the decomposer, the sodium-mercury amalgam is reacted with water to form sodium hydroxide, hydrogen and mercury. Water is fed to the decomposer through inlet 35. Hydrogen is withdrawn from outlet 36 and sodium hydroxide through outlet 37. Mercury flows out of the decomposer through conduit 38 and is recycled to the cell by pump 39 via conduit 10. The above is intended to be a description of a typical mercury cell and its operation and this invention is applicable to substantially all types of horizontal mercury cells.

In the operation of cells of this type equipped with dimensionally stable anodes, the anode faces are positioned as closely as practicable to the surface of the flowing mercury cathode. The electrolyte gap is usually between 1.5 and 3.5 mm. and for best efficiency of the electrolysis process, it should be kept as small as practicable, taking into consideration all operating conditions of the particular cell. However, due to ripples, uneven-flow and other conditions, the mercury surface does not always remain smooth and when, for any reason, the mercury surface rises to where it touches an anode face, a short circuit is formed between the mercury and the anode. With oxide coated anodes, the face of the anode is not immediately wetted by the mercury and as the mercury is constantly flowing from end to end of the cell, these short circuits are usually of short duration and are usually automatically broken by the mercury flow or other conditions within the cell. They are, however, of sufficient duration to cause small explosions or pippings when, in the presence of a spark, small amounts of hydrogen and chlorine react together. When these explosions rupture the 'cell cover, it is not possible to operate the cell until the-cover is repaired and because portions of the cover are beneath the anode supporting frames, or beneath the anode bus bars or other portions of the cell, the ruptures are not always easily accessible for repair. 7

FIGS. 1 to 13 show various means by which the cell covers are provided with weakened portions, or automatic venting valves, which are in accessible places, whereby the ruptures occur at points which are easily accessible for ready repair.

FIGS. 1 and 2 show a series of holes 50 through the cell cover 17 which are covered by flap valves 51, pivotally secured to the cover by vulcanization or suitable adhesive, so that in the event of an explosion near a hole 50 the valve 51 will pop up as indicated by the dotted line position in FIG. 2 and then fall back into place. As these cells normally operate under a slight vacuum in the chlorine withdrawing system, the reseating of the valve 51 over the hole 50 will normally be sufficiently tight to seal the opening. The valves 51 may, however, be weighted to cause them to return to their sealing position when the explosive force has dissipated itself, and may be provided with surrounding extending edges or lips to provide a better seal.

In FIG. 3, elongated flap valves 53 pivoted at 53a cover holes 52 in the cell cover 17. The flap valves 53 may be held in sealing position over the holes 52 by easily breakable adhesive strips 54 which when ruptured can be removed and replaced. The flap valves 51 and 53 and the corresponding holes 50 and 52 are located where they are easily accessible.

FIGS. 4 and show holes 55 covered by covers 56 which are thinner or weaker than the cell cover 17 and are replaceably secured in place by plastic rivets 57 or by adhesive, so that in the event of rupture the covers 56 may be removed and replaced.

FIG. 6 shows weakened sections 17a in the cell cover 17 weakened by notches 17b, which will blow out to relieve the force of an explosion and which can be scaled back in place by adhesive strips 58. The weakened sections 17a may be in the form of triangles, rectangles, round or any other desired shape in the bottom or the top surface of cover 17. They are preferably in the bottom surface of cover 17 so as to present a smooth top sur face for the application of adhesive strips 58.

FIG. 7 illustrates a weighted valve 59 fitting into a hole 60 in the cover 17. The valve 59 has a frustoconical weighted body 59a, secured by an arm 59b to the cover 17 by a flexible strip or strips 590 secured by adhesive or vulcanization to the cover 17. When an explosive force lifts valve 59 from its seat in hole 60, the weight of the valve tends to return it to its seat as soon as the force of the explosion has dissipated.

FIG. 8 shows a water seal 61 in an insert 62 in the cell cover 17. This can be a flexible cell cover of rubber or other plastic material, or a rigid cover of rubber lined steel, hard rubber or other material, or a steel cover with flexible rubber connections to the walls of the cell. The chlorine outlet 63-6311 is provided with an enlarged inverted chamber 64 with a base with a plurality of openings 65 therein. Water is placed in the seal 61. If an explosion generates more force than the chlorine outlet 6311 can relieve, the water in seal 61 is blown out and the force of the explosion is relieved by the openings 65 in the base of chamber 64. To again seal the opening 63 against explosive forces, it is only necessary to add water to the seal 61.

FIG. 9 shows a similar water seal 61 in an insert 62 in the cell cover 17 for use with either a flexible or rigid cell cover. In this embodiment, a lid 6'6 with a depending rim 67 is pivoted on a plastic pivot 69 secured to the cell cover 17. Openings 67a in the rim 67 permit water to flow from the inside to the outside of the rim. The force of an explosion, first, blows the water out of the seal 61 and if this does not sufficiently relieve the pressure, the lid 66 is raised by the force of the explosion. To restore the seal, the rim 67 is inserted in the seal 61 and water is added to the seal.

FIG. and 11 show openings 69 in the cell cover 17 into which rubber balloons 70 may be inserted and filled with air or any suitable gas to partially inflate the balloons to seal the opening 69. A calibrated scale 71 on the balloons permits the balloons to be inserted with the desired projection into the cell and shows how far the balloons should be inserted into the cell before they are inflated to seal the openings 69. Valves 70 permit the balloons to be inflated after they are partially inserted into the cell. For minor explosions, the portions of the balloons inside the cell can partially compress, as shown by the dotted outline 70a, and the portions outside the cell can expand, as shown by the dotted outline, to relieve the explosion without rupturing the balloon. For larger explosions, the balloons 70 will rupture and be blown out of the openings 69, and must be replaced with new balloons.

FIG. 12 indicates the usual connection of the flexible cell cover to the side and end walls of the cell, in which the flexible cell cover 17 is bolted to the side and end walls of the cell by a bolted strip of metal 24 by bolts 24b. FIG. 13 (and also FIG. 1) shows sections 24a of the metal strips 24 loosely mounted on bolts 24b, so that in the event of an explosion, the sections of the cell cover under the sections 24a of the strips 24 can raise on bolts 24b to relieve the force of the explosion, after which the sections of the cell cover raised by the explosion are pressed back or fall back into sealing contact with the side or end Walls of the cell by the weight of the metal in the movable sections 24a.

Another method of weakening the cell covers whereby the force of the explosions may be vented and the cell cover readily restored to chlorine-tight condition is to tighten the lock nuts 30b, which clamp the cell cover 17 to the titanium sleeves 27c, only to the extent necessary to make these joints chlorine-tight and sufficiently loose that the explosions will blow out through the holes arounud the lead-ins 27. In order to restore the cover to chlorine leak-proof conditions, the lock nuts 30b are loosened, the cover pushed back into place and the nuts retightened. We have found that the use of an automatic pressure wrench adjusted to exert a pressure of 30 ft. lbs. on the lock nuts 30b is suitable to insure that the lock nuts 30b will be tightened enough to seal the cover around the lead-ins 27 but still sufliciently loose to relieve explosions without rupture of the cell cover. This method relieving the explosion pressures is not as satisfactory as the other methods described but may be used with the other methods or without.

While we have set forth preferred embodiments of our invention to enable those skilled in the art to understand and practice our invention, it will be understood that the invention is not limited to these embodiments and that the invention may be embodied in various modifications without departing from the spirit of the in vention. For example, this new type of cover may be used on other types of electrolytic cells in which it has previously been the practice to suspend the anodes from the cover of the cell. One example of such a cell is the vertical diaphragm cell described in the patent to Stuart 2,370,087. Other similar cells may also be suitable for the use of our new cover. It may be used with graphite anodes but is more appropriately used with dimensionally stable anodes.

What is claimed is:

1. A horizontal flowing mercury cathode electrolysis cell having a cell trough, dimensionally stable metal anodes, a flowing mercury cathode, and a flexible cell cover provided with easily repairable and easily accessible means to vent explosions through selected areas of said cell cover without disturbing the anodes.

2. The cell of claim 1 wherein the flexible cell cover is provided with weakened areas of reduced cross-section which will preferentially rutpure before the cell cover.

3. A horizontal flowing mercury cathode electrolysis cell having a cell trough, dimensionally stable metal anodes, a flowing mercury cathode and a flexible cell cover provided witheasilyaccessibleand resettable valve means which will automatically open to vent explosions through selected areas of said cell cover without disturbing theanodes. 7

The cell of claim 3 wherein the valve means is a flap valve held in position by easily breakable adhesive strips. k y Y v I.

"5. The cell of claim 3.wher ein the valvemeans is a weighted valve tending. to reseat itself.

6.. The cell coverof claim 3 in which the valves which automatically open also automatically close. H I 7. The method of. operating an electrolysis cell havingdimensionally, stable metal anodes to reduce damage to the cover from explosionswithin the cellwhich comprises. providing a -flexible cover for said cells having weakened areas ,,with a ,reduced 'cross-section ineasily accessible portions of saidcover, preferentially venting explosions in the cell through said weakened areas and repairing weakened areas through which explosions have been vented. 1

.. 8.1. The method of operating an electrolysis tcell hav- ,ing dimensionally stable metal anodes to reduce damage t o the ,cell cover from explosions within the cell which comprises providing a flexible cell cover provided with easily accessible and easily resetta ble v valve means which will open before the cell cover will rupture,preferential- -ly venting explosions in the cell through said valve means and resetting the valve means after venting Of, the explosion.

Refere nces Cited UNITED STATES PATENTS 1 184,938 8/1922 Great Britain -204-.-99

JOHN H. MACK, Primary Examiner D; VALENTINE, Assistant Examiner Us. (:1, XR. 

